There is known a drive system for a vehicle, which includes a differential mechanism functioning as a transmission having a differential function, and an automatic transmission operable in a step-variable shifting state, and which is arranged to transmit an output of a drive power source to drive wheels of the vehicle. Generally, the overall speed ratio of the vehicular drive system of this type is determined by the speed ratios of those two transmission mechanisms.
As an example of the differential mechanism functioning as a transmission having a differential function, there is known a drive system which includes a differential device operable to distribute an output of an engine to a first electric motor and an output shaft, and a second electric motor which disposed between the differential device and the drive wheels, and which is operable as a continuously controlled continuously variable transmission. Examples of this type of drive system include drive systems for a hybrid vehicle as disclosed in Patent Documents 1-6. In this hybrid vehicle drive system, the differential mechanism is constituted by a planetary gear set, for example, and a major portion of a drive force of the engine is mechanically transmitted to the drive wheels through the differential function of the differential mechanism, while the rest of the drive force is electrically transmitted from the first electric motor to the second electric motor, through an electric path therebetween, so that the differential mechanism functions as an electrically controlled continuously variable transmission the speed ratio of which is electrically variable, thereby making it possible to drive the vehicle under the control of a control device, with the engine kept in an optimum operating state with an improved fuel economy.
The overall speed ratio of vehicular drive system including the two transmission mechanisms indicated above is determined by the speed ratio of the differential device having the differential function, and the speed ratio of the automatic transmission. When the differential mechanism is placed in its continuously-variable shifting state, the drive system as a whole functions as the electrically controlled continuously variable transmission, like a drive system including only the differential device, and the vehicle is driven with the engine kept in its optimum operating state.                Patent Document 1: JP-2003-301731A        Patent Document 2: JP-2003-130203A        Patent Document 3: JP-2003-127681A        Patent Document 4: JP-2004-130202A        Patent Document 5: JP-11-198668A        Patent Document 6: JP-11-198668A        